Master cylinder apparatus

ABSTRACT

A master cylinder apparatus for a BBW system is provided, which eliminates a risk of brake delay, even with the use of a plunger-type master cylinder, and which therefore is able to achieve a reduction in size of a master cylinder. The master cylinder apparatus comprises a tandem-type master cylinder connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from a fluid pressure chamber in the master cylinder. The master cylinder is arranged in the form of a plunger-type master cylinder in which cup seals are provided on an inner surface of a bore of the cylinder body. A spring force of a second return spring for a secondary piston is set to be larger than that of a first return spring for a primary piston. A retracted position of the secondary piston is limited by a stopper pin extending through an oblong hole formed in the secondary piston.

BACKGROUND OF THE INVENTION

The present invention relates to a brake system for an automobile. Morespecifically, the present invention relates to a master cylinderapparatus used in a brake fluid pressure controlling system forelectrically controlling a fluid pressure supplied to wheel cylinders;i.e., a so-called brake-by-wire (BBW) system.

A master cylinder apparatus for a BBW system comprises a master cylinderadapted to be connected to wheel cylinders through fail-safe valves anda stroke simulator for ensuring a desired stroke of a brake pedal uponreceiving a brake fluid introduced from the master cylinder. In theevent of failure of the BBW system, the fail-safe valves are opened, andthe fluid pressure generated in the master cylinder is supplied to thewheel cylinders.

This type of related art is disclosed in, for example, U.S. Pat. No.6,192,685 B1.

However, the master cylinder apparatus of U.S. Pat. No. 6,192,685 B1 isconfigured such that supply of brake fluid from a reservoir to aprimary-side pressure chamber and a secondary-side pressure chamber iscontrolled by means of a center valve. Consequently, axial dimensions ofthe primary piston and the secondary piston are large, and an overallsize of a master cylinder is also large.

To counter this problem, it has been suggested to use a plunger-typemaster cylinder in which cup seals are provided on an inner surface of abore of a cylinder body, so as to seal outer circumferential surfaces ofa primary piston and a secondary piston, each of which is subject to asliding movement in the cylinder body. In the case of a master cylinderapparatus for a BBW system, it is generally required to set a springforce of a return spring for biasing a secondary piston to be largerthan that of a return spring for biasing a primary piston, so as tolimit a force that a driver is required to apply to a brake pedal. Insuch a configuration, since a spring force for biasing the secondarypiston must be made large, and a return position (a retracted position)of the secondary piston during non-braking is therefore variable, apossibility exists that in the event of a failure of the BBW system, aninvalid stroke for operating the secondary piston will be large,resulting in a delay in braking.

SUMMARY OF THE INVENTION

The present invention has been made with a view to overcoming thedrawbacks of the prior art, as stated above. It is therefore an objectof the present invention to provide a master cylinder apparatus whicheliminates a risk of brake delay in the event of failure of a BBWsystem, even with the use of a plunger-type master cylinder, and whichtherefore is able to safely achieve a reduction in size of a mastercylinder.

In order to achieve the above-mentioned object, the present inventionprovides a master cylinder apparatus comprising:

-   -   a tandem-type master cylinder adapted to be connected to wheel        cylinders through fail-safe valves; and    -   a stroke simulator for ensuring a desired stroke of a brake        pedal upon receiving a brake fluid introduced from a        primary-side fluid pressure chamber formed in the master        cylinder,    -   wherein:    -   the tandem-type master cylinder comprises:    -   a cylinder body;    -   a primary piston and a secondary piston arranged in an axial        direction of the cylinder body and being capable of sliding        movement in the cylinder body;    -   the primary-side fluid pressure chamber, which is defined        between the primary piston and the secondary piston;    -   a secondary-side fluid pressure chamber defined between the        secondary piston and the cylinder body;    -   a first circumferential groove and a second circumferential        groove formed in an inner surface of a bore of the cylinder        body, the first circumferential groove and the second        circumferential groove being spaced apart from each other in the        axial direction of the cylinder body;    -   a first cup seal provided in the first circumferential groove        and adapted to seal an outer circumferential surface of the        primary piston;    -   a second cup seal provided in the second circumferential groove        and adapted to seal an outer circumferential surface of the        secondary piston;    -   a first return spring provided in the primary-side fluid        pressure chamber so as to bias the primary piston in a direction        for retraction;    -   a second return spring provided in the secondary-side fluid        pressure chamber so as to bias the secondary piston in the        direction for retraction; and    -   a limiting means provided in the cylinder body so as to limit a        retracted position of the secondary piston.

In this master cylinder apparatus, the second return spring for biasingthe secondary piston may have a larger spring force than the firstreturn spring for biasing the primary piston. The limiting means maycomprise a stopper pin which extends across the bore of the cylinderbody, the stopper pin being provided in an oblong hole extending throughthe secondary piston.

In the master cylinder apparatus arranged as mentioned above, there isprovided a limiting means adapted to limit a retracted position of asecondary piston; as a result of which a retracted position of thesecondary piston remains constant, even in a case that a spring force ofa return spring for biasing the secondary piston is larger than that ofa return spring for biasing a primary piston. By this means, an invalidstroke of the secondary piston can be markedly reduced. Further, by useof a plunger-type master cylinder, an axial dimension of a mastercylinder can also be reduced.

Since the retracted position of the secondary piston is limited by thelimiting means, an invalid stroke of the secondary piston is reduced.Therefore, in the event of failure of the BBW system, brake delay doesnot occur, and the apparatus is made highly reliable. Further, since anaxial dimension of a master cylinder is reduced by use of a plunger-typemaster cylinder, the master cylinder apparatus is easily mountable on avehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an entire structure of a mastercylinder apparatus according to an embodiment of the present invention.

FIG. 2 is a side view showing the entire structure of the mastercylinder apparatus of FIG. 1.

FIG. 3 is a cross-sectional view showing a structure of a mastercylinder of the master cylinder apparatus of FIG. 1.

FIG. 4 is an enlarged cross-sectional view of a part of the mastercylinder of FIG. 3.

FIG. 5 is a cross-sectional view showing a primary-side cup seal inassembled position.

FIG. 6 is a cross-sectional view showing a secondary-side cup seal inassembled position.

FIG. 7 is a cross-sectional view showing a structure of anopening/closing means of the master cylinder apparatus of FIG. 1.

FIG. 8 is a plan view of the opening/closing means shown in FIG. 7, asviewed from inside a bore of the master cylinder.

FIG. 9 is a perspective view of a rocking lever of the opening/closingmeans of FIG. 7.

FIG. 10 is a cross-sectional view showing an operation of theopening/closing means of FIG. 7.

FIG. 11 is a schematic diagram showing a structure of a stroke sensor ofthe master cylinder apparatus of FIG. 1.

FIG. 12 is a schematic diagram showing a modified example of a strokesensor of the master cylinder apparatus.

FIG. 13 is a schematic diagram showing another modified example of astroke sensor of the master cylinder apparatus.

FIG. 14 is a graph indicating a relationship between the stroke and theangle of rotation in each of the stroke sensors shown in FIGS. 11 to 13.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, an embodiment of the present invention is described, withreference to the accompanying drawings.

FIGS. 1 and 2 show the entire structure of a master cylinder apparatusaccording to an embodiment of the present invention. A master cylinderapparatus 1 is used in a BBW system described above. The master cylinderapparatus 1 is connected to wheel cylinders (not shown) throughfail-safe valves 2A and 2B. The master cylinder apparatus 1 comprises atandem-type master cylinder 4 for generating a fluid pressurecorresponding to a force applied to a brake pedal 3, and a strokesimulator 5 for ensuring a desired stroke of the brake pedal 3. Thestroke simulator 5 is externally mounted on a cylinder body 10 of themaster cylinder 4. A first fluid pressure chamber (a primary-side fluidpressure chamber) 13 is defined between a primary piston 11 and asecondary piston 12 provided in the master cylinder 4. When the brakepedal 3 is operated, a brake fluid in the first fluid pressure chamber13 is introduced into the stroke simulator 5, to thereby ensure adesired stroke of the brake pedal 3.

The master cylinder apparatus 1 further comprises an opening/closingmeans 7 and a stroke sensor 8. The opening/closing means 7 is providedin a simulator passage 6 which allows communication between the firstfluid pressure chamber 13 in the master cylinder 4 and the strokesimulator 5. The stroke sensor 8 is adapted to detect a stroke of theprimary piston 11 (a piston stroke), which moves in the master cylinder4 in accordance with the movement of the brake pedal 3. The BBW systemcomprises a fluid pressure controlling means including a fluid pressuresource, a fluid pressure control valve and an electronic control unit,etc., in addition to the master cylinder apparatus 1. Normally, thefluid pressure controlling means controls a fluid pressure supplied tothe wheel cylinders, on the basis of a detection signal emitted from thestroke sensor 8.

The master cylinder 4 is arranged in the form of a plunger-type mastercylinder. The cylinder body 10 is arranged in the form of a cylinderhaving one end closed, as shown in FIG. 3. The primary piston 11 and thesecondary piston 12 are slidably provided in a bore 14 of the cylinderbody 10.

A forward end (an end for insertion into the bore 14) of the primarypiston 11 forms a cup-like portion 11 a. The first fluid pressurechamber 13 is defined between the cup-like portion 11 a of the primarypiston 11 and the secondary piston 12. A forward end of the secondarypiston 12 also forms a cup-like portion 12 a. A second fluid pressurechamber (a secondary-side fluid pressure chamber) 15 is defined betweenthe cup-like portion 12 a and the closed end of the cylinder body 10.The cylinder body 10 includes a first discharge port 16 for supplyingbrake fluid from the first fluid pressure chamber 13 to thecorresponding wheel cylinders, and a second discharge port 17 forsupplying brake fluid from the second fluid pressure chamber 15 to thecorresponding wheel cylinders. The first discharge port 16 opens into alongitudinal groove 18 formed in an inner surface of the bore 14 of thecylinder body 10; and the second discharge port 17 opens into alongitudinal groove 19 formed in the inner surface of the bore 14 of thecylinder body 10.

A first return spring 20 is provided between a bottom of the cup-likeportion 11 a of the primary piston 11 and the secondary piston 12. Asecond return spring 21 is provided between a bottom of the cup-likeportion 12 a of the secondary piston 12 and the closed end of thecylinder body 10. Normally, spring forces of the first and second returnsprings 20 and 21 bias each of the pistons 11 and 12 in a direction awayfrom the bore 14.

The spring force of the second return spring 21 for biasing thesecondary piston 12 is set to be larger than that of the first returnspring 20 for biasing the primary piston 11.

A rear end portion of the cylinder body 10 is connected to a pistonguide 23 in the form of a cylinder having one end closed, by means of aretaining member 22 threadably engaged with the rear end portion of thecylinder body 10. A bottom plate of the piston guide 23 prevents theprimary piston 11 from separating from the bore 14, while limiting aretracted position of the primary piston 11. A retracted position of thesecondary piston 12 is limited by a stopper pin (a limiting means) 25which is inserted into a diametrical hole (an oblong hole) 24 extendingthrough a solid portion of the secondary piston 12. As shown in FIG. 4,the stopper pin 25 extends across the bore 14, with a base end portionthereof being threadably engaged with a wall of the cylinder body 10.

A rear end (opposite to the end for insertion) of the primary piston 11includes a recess 11 b extending along the axis of the primary piston11. An input shaft 26 extending from the brake pedal 3 is inserted intothe recess 11 b. The input shaft 26 is locked in the recess 11 b in astate such that a spherical portion 26 a formed at a forward end of theinput shaft 26 abuts against a bottom part of the recess 11 b. Theprimary piston 11 is adapted to advance toward a closed end of the bore14 under a force applied from the brake pedal 3 through the input shaft26.

The inner surface of the bore 14 of the cylinder body 10 includes twoannular grooves 27 and 28. The annular groove 27 faces the primarypiston 11, and the annular groove 28 faces the secondary piston 12. Areservoir port 30 communicating with a reservoir 29 mounted on the topof the cylinder body 10 opens into the annular groove 27. A reservoirport 31 communicating with the reservoir 29 opens into the annulargroove 28. The cup-like portion 11 a of the primary piston 11 includes asupply port 32, and the cup-like portion 12 a of the secondary piston 12includes a supply port 33. When the primary piston 11 and the secondarypiston 12 are located at their respective retracted positions, thesupply port 32 and the supply port 33 are open to the annular groove 27and the annular groove 28, respectively. In this state, brake fluid issupplied from the reservoir 29 to the first fluid pressure chamber 13and the second fluid pressure chamber 15.

A pair of cup seals 34 and 35 are provided on the inner surface of thebore 14 of the cylinder body 10, with the primary-side annular groove 27being disposed therebetween. Further, on the inner surface of the bore14 of the cylinder body 10, a pair of cup seals 36 and 37 are provided,with the secondary-side annular groove 28 being disposed therebetween.

Of the cup seals on the primary side, the cup seal 34 located on a sideof an open end of the bore 14 serves to seal the first fluid pressurechamber 13 from the outside. Of the cup seals on the secondary side, thecup seal 36 located on a side of the open end of the bore 14 serves toprevent communication between the first fluid pressure chamber 13 andthe second fluid pressure chamber 15.

On the other hand, of the cup seals on the primary side, the cup seal 35located on a side of the closed end of the bore 14 serves to preventflow of the fluid from the first fluid pressure chamber 13 to theannular groove 27 communicated with the reservoir 29. Of the cup sealson the secondary side, the cup seal 37 located on a side of the closedend of the bore 14 serves to prevent flow of the fluid from the secondfluid pressure chamber 15 to the annular groove 28 communicated with thereservoir 29.

The primary-side cup seals 34 and 35 and the secondary-side cup seals 36and 37 are respectively provided in annular grooves formed in the innersurface of the bore 14 of the cylinder body 10. The cup seal 35 on aside of the closed end of the bore 14 is provided in an annular groove38, which is communicated with the longitudinal groove 18 formed in theinner surface of the bore 14. The cup seal 37 on a side of the closedend of the bore 14 is provided in an annular groove 39, which iscommunicated with the longitudinal groove 19 formed in the inner surfaceof the bore 14. As shown in FIG. 5, the longitudinal groove 18, which iscommunicated with the annular groove 38 in which the primary-side cupseal 35 is provided, is shallower than the annular groove 38. That is,an outer circumferential edge of the cup seal 35 abuts against a frontwall surface 38 a of the annular groove 38 to limit a flow of the fluidfrom the annular groove 27 behind the cup seal 35 to the first fluidpressure chamber 13. In contrast, as shown in FIG. 6, the longitudinalgroove 19, which is communicated with the annular groove 39 in which thesecondary-side cup seal 37 is provided, has a depth equal to or slightlylarger than the depth of the annular groove 39, thus allowing flow ofthe fluid from the annular groove 28 behind the cup seal 37 to thesecond fluid pressure chamber 15.

As shown in FIG. 1, the stroke simulator 5 comprises a simulator body 40having a stepped configuration including a small-diameter portion 40 a,an intermediate-diameter portion 40 b and a large-diameter portion 40 c.External threads are formed in an outer circumferential surface of theintermediate-diameter portion 40 b. On the other hand, a boss portion 42having a stepped inner surface defining a fitting opening 41 isprojected from the cylinder body 10 of the master cylinder 4. Alarge-diameter portion of the fitting opening 41 has internal threads.The simulator body 40 of the stroke simulator 5 is directly connected tothe cylinder body 10 by threadably engaging the intermediate-diameterportion 40 b with the fitting opening 41 of the cylinder body 10. Thus,the simulator body 40 of the stroke simulator 5 is mounted on anexterior surface of the cylinder body 10. To connect the simulator body40 to the cylinder body 10, the small-diameter portion 40 a at a forwardend of the simulator body 40 is press-fitted into a small-diameterportion of the fitting opening 41 through a seal member 43 (see FIG. 7).

The simulator passage 6 which communicates the first fluid pressurechamber 13 of the master cylinder 4 with the stroke simulator 5comprises a port 50 (described later) formed at a bottom end of thefitting opening 41 of the cylinder body 10, a fluid passage 51 in theopening/closing means 7 and a fluid passage 52 formed in the simulatorbody 40 (FIGS. 3 and 7).

As indicated in FIG. 1, the simulator body 40 of the stroke simulator 5includes a bore 44 having an end wall. A piston 46 is slidably providedin the bore 44 through a cup seal 45. A pressure chamber S is definedbetween a forward end (an end for insertion into the bore 44) of thepiston 46 and the end wall of the bore 44, which pressure chamber issealed by the cup seal 45. The fluid passage 52 forming the simulatorpassage 6 opens into the pressure chamber S. The large-diameter portion40 c of the simulator body 40 has a hollow portion. The bore 44 isextended so as to form a cylindrical extension 44 a in the hollowportion of the large-diameter portion 40 c. The large-diameter portion40 c of the simulator body 40 has an open end on a side opposite to thesmall-diameter portion 40 a. The open end of the large-diameter portion40 c is closed by a cover plate 40′. A spring bearing 48 is provided ata distal end of the cylindrical extension 44 a of the large-diameterportion 40 c, so as to face the cover plate 40′. A first spring 47 isdisposed between the spring bearing 48 and the cover plate 40′. One endof the first spring 47 is seated on the cover plate 40′. The other endof the first spring 47 is received by the spring bearing 48. Further, asecond spring 49 having a smaller spring force than the first spring 47is provided inside the cylindrical extension 44 a. The second spring 49is interposed between the spring bearing 48 and a cup-like surface ofthe piston 46 and normally biases the piston 46 in an upward direction.In the stroke simulator 5, when the fluid pressure in the pressurechamber S rises, the piston 46 first moves downward against the springforce of the second spring 49 and abuts against the spring bearing 48.Thereafter, the piston 46 moves downward against the spring force of thefirst spring 47.

As is clearly shown in FIGS. 7 to 10, the opening/closing means 7comprises a poppet valve 54 for opening and closing the fluid passage 52(the simulator passage 6) in the simulator body 40 and a rocking lever55 which is movable together with the secondary piston 12 in the mastercylinder 4 to open and close the poppet valve 54. The poppet valve 54and the rocking lever 55 are assembled in a casing 56, to form a unit.To assemble the simulator body 40 to the fitting opening 41 of the bossportion 42 of the cylinder body 10, the unit is connected between thecylinder body 10 and the simulator body 40 through a washer 57. Asindicated in FIG. 8, the port 50 forming the simulator passage 6 on aside of the cylinder body 10 has an oblong (non-circular) form, whichextends in an axial direction of the bore 14 of the cylinder body 10.The casing 56 includes a C-shaped projection 56 a projected upwardlytherefrom. The C-shaped projection 56 a is fitted into the oblong port50 while being located close to the front side (on a side of the closedend of the bore 14) of the oblong port 50. Thus, by means of the oblongport 50, the casing 56 is prevented from rotating.

The poppet valve 54 comprises a valve seat 61, a valve body 62 adaptedto move to and away from the valve seat 61 and a valve spring 63 whichnormally biases the valve body 62 in a direction for closing the valve.The valve seat 61 is formed at the bottom of a recess 60 formed in thesimulator body 40, so as to surround an opening of the fluid passage 52.The fluid passage 52 is closed when the valve body 62 is seated on thevalve seat 61, and is open when the valve body 62 is separated from thevalve seat 61. The valve spring 63 has one end engaged with the casing56, and normally biases the valve body 62 in the direction for closingthe valve. The valve body 62 is slidably fitted into a through-hole 56 bformed in the casing 56, with a lower end thereof being connected to anelastic member 64 capable of intimate contact with the valve seat 61. Anupper end portion of the valve body 62 includes a constricted portion 62a engageable with the rocking lever 55. The fluid passage 51 in theopening/closing means 7, which forms the simulator passage 6, is aninclined passage formed in the casing 56, with one end thereof openinginto the through-hole 56 b. Therefore, when the poppet valve 54 is open,as shown in FIG. 7, brake fluid in the first fluid pressure chamber 13of the master cylinder 4 is supplied to the stroke simulator 5 throughthe port 50 in the cylinder body 10, the fluid passage 51 in the casing56, the recess 60 around the valve body 62 and the fluid passage 52 inthe simulator body 40. The material of the elastic member 64 is notparticularly limited, as long as it is capable of intimate contact withthe valve seat 61. Various materials, such as rubber, resin, etc., maybe used for the elastic member 64.

As is clearly shown in FIG. 9, the rocking lever 55 comprises a shaftportion 65 supported by a shaft bearing portion formed in the casing 56,a claw portion 66 extending radially outwardly from the shaft portion 65and a columnar portion 67 extending from the shaft portion 65substantially at a right angle relative to the claw portion 66. The clawportion 66 is engageable with the constricted portion 62 a of the valvebody 62. When the rocking lever 55 is supported by the casing 56, anupper end of the columnar portion 67 of the rocking lever 55 extendsthrough the port 50 of the cylinder body 10 into the bore 14. A rear endportion of the secondary piston 12 includes an annular groove 68. Theupper end of the columnar portion 67 is positioned within the annulargroove 68.

When the secondary piston 12 is located at the retracted position, afront wall surface 69 of the annular groove 68 is positioned close tothe rear side of the oblong port 50. In this state, the columnar portion67 of the rocking lever 55 is rotated by the front wall surface 69 in aclockwise direction in FIG. 7, and finally abuts against the front wallsurface 69 and maintains an upright position (FIG. 7). Consequently, thevalve body 62 of the poppet valve 54 is lifted by the claw portion 66and, as shown in FIG. 7, the fluid passage 52 (the simulator passage 6)on a side of the stroke simulator 5 is opened. When the secondary piston12 advances from the retracted position, the front wall surface 69 movesin a leftward direction in FIG. 7, to thereby release the force of therocking lever 55 lifting the valve body 62 of the poppet valve 54. Sincethe valve body 62 is biased in the valve-closing direction by means ofthe valve spring 63, the rocking lever 55 rocks about the shaft portion65, and the fluid passage 52 is closed, as shown in FIG. 10.

As shown in FIGS. 1 and 3, the stroke sensor 8 is provided in a cover 70connected to a flange portion 10 a at the rear end portion of thecylinder body 10. The stroke sensor 8 comprises a sensor body 71containing a rotation angle detector (not shown), a rotary shaft 72extending from the rotation angle detector downward beyond the lowersurface of the sensor body 71, a sensor arm 73 having one end fixedlyconnected to the rotary shaft 72, and a sensor pin 75. The sensor pin 75extends upward from the rear end of the primary piston 11 and extendsthrough a slit 74 formed in the piston guide 23 toward the sensor body71.

As is clearly shown in FIG. 11, an oblong hole 76 is formed in the otherend of the sensor arm 73. An upper end portion of the sensor pin 75 isinserted into the oblong hole 76. The sensor pin 75 is adapted tolinearly move, together with the primary piston 11, in the slit 74formed in the piston guide 23. The oblong hole 76 of the sensor arm 73has a length sufficient for ensuring a linear movement of the sensor pin75, and has a width sufficient for ensuring a smooth motion of thesensor pin 75. The sensor arm 73 is biased in a counterclockwisedirection as viewed in FIG. 11, by means of a biasing means (not shown).By this arrangement, a wall surface 76 a on one side of the oblong hole76 is always pressed against the sensor pin 75. That is, the sensor pin75 is adapted to perform linear movement without play in the oblong hole76 of the sensor arm 73. Therefore, an amount of linear movement of theprimary piston 11 can be accurately converted to an amount of rotationof the rotary shaft 72. In this case, as indicated by a solid line inFIG. 14, a substantially linear relationship exists between the amountof linear movement of the primary piston 11 and the angle of rotation ofthe rotary shaft 72, as a result of which accurate sensing can be stablyconducted over an entire length of a stroke.

Instead of the sensor arm 73 having a straight form shown in FIG. 11, aV-shaped sensor arm 73′ as shown in FIG. 12 or a curved sensor arm 73″as shown in FIG. 13 may be employed.

The sensor arm 73′ is bent in a direction outward relative to a lineconnecting the rotary shaft 72 and the sensor pin 75, and the sensor arm73″ is curved outward relative to the line connecting the rotary shaft72 and the sensor pin 75. In the sensor arm 73″, the oblong hole 76 isalso curved. When the V-shaped sensor arm 73′ is used, as indicated by adotted line in FIG. 14, high resolution can be obtained during earlyperiods of a stroke while resolution near the end of the stroke issomewhat compromised. When the curved sensor arm 73″ is used, asindicated by a one-dot chain line in FIG. 14, a resolution obtained isintermediate between those of the sensor arms 73 and 73′. However, therelationship between the angle of rotation and the stroke becomeslinear, with the result that data processing using a sensor output canbe easily conducted.

Hereinbelow, an operation of the master cylinder apparatus 1 isdescribed. The master cylinder apparatus 1 is connected to a vehiclebody using a stud bolt 80. The stud bolt 80 extends from a front side ofthe flange portion 10 a of the cylinder body 10 through the cover 70that accommodates the stroke sensor 8.

First, when the BBW system is normally operated, the fail-safe valves 2Aand 2B are closed. Therefore, dependent on a force applied to the brakepedal 3, the primary piston 11 advances in a leftward direction asviewed in FIGS. 1 and 3, and a fluid pressure corresponding to the inputfrom the brake pedal 3 is generated in the first fluid pressure chamber13. In this instance, the poppet valve 54 of the opening/closing means 7is open due to engagement with the secondary piston 12 at the retractedposition (FIG. 7). Therefore, the brake fluid in the first fluidpressure chamber 13 passes through the port 50 of the cylinder body 10,the fluid passage 51 in the opening/closing means 7 and the fluidpassage 52 in the simulator body 40 and is supplied to the pressurechamber S in the stroke simulator 5.

Although the secondary piston 12 marginally advances in accordance withthe rise in pressure in the first fluid pressure chamber 13, the poppetvalve 54 is not caused to close as a result of this movement of thesecondary piston 12.

When brake fluid is introduced into the pressure chamber S of the strokesimulator 5, the piston 46 first moves downward against the spring forceof the second spring 49 having a force smaller than that of the firstspring 47, so as to ensure an appropriate initial stroke of the brakepedal 3. After the piston 46 abuts against the spring bearing 48, thepiston 46 moves downward against the spring force of the first spring47, which is larger than that of the second spring 49, to thereby ensurea desired stroke of the brake pedal 3. As the piston 46 moves downward,a reactive force acting on the brake pedal 3 increases. In this way, aso-called pedal resistance is generated, to thereby obtain an optimumpedal-feel. Meanwhile, the amount of movement of the primary piston 11is monitored by the stroke sensor 8. Based on a signal emitted from thestroke sensor 8 (a piston stroke), the electronic control unit in theBBW system controls the fluid pressure supplied to the wheel cylinders,to thereby obtain a desired braking force.

The braking force is controlled on the basis of a piston stroke in theabove-mentioned manner. Thus, if a fluid is caused to repeatedly flowfrom behind the cup seal 35 to the first fluid pressure chamber 13 (abackside flow) under repetitive operation of the brake pedal 3, areactive force acting on the brake pedal 3 increases, which makes itdifficult to obtain an appropriate braking force in correspondence withoperation of the brake pedal. However, in this embodiment, as indicatedin FIG. 5, the outer circumferential edge of the primary-side cup seal35 abuts against the front wall surface 38 a of the annular groove 38,thus preventing the backside flow. Therefore, there is no flow of fluidfrom behind the primary-side cup seal 35 to the first fluid pressurechamber 13, so that a desired braking force can be stably obtained, evenwhen the brake pedal 3 is repeatedly operated.

Next, description is made with regard to an operation of the mastercylinder apparatus 1 in the event of failure of the BBW system. In thiscase, the fail-safe valves 2A and 2B are opened, and the master cylinder4 is fluidly connected to the wheel cylinders. Then, the primary piston11 advances in accordance with a force applied to the brake pedal 3, soas to increase a fluid pressure in the first fluid pressure chamber 13.The brake fluid in the first fluid pressure chamber 13 flows from thefirst discharge port 16 through the fail-safe valve 2A to thecorresponding wheel cylinders. On the other hand, the secondary piston12 also advances according to an increase in the fluid pressure in thefirst fluid pressure chamber 13, and brake fluid in the second fluidpressure chamber 15 is supplied from the second discharge port 17through the fail-safe valve 2B to the corresponding wheel cylinders.

As the secondary piston 12 advances, as indicated in FIG. 10, therocking lever 55 of the opening/closing means 7 rocks about the shaftportion 65, and the valve body 62 of the poppet valve 54 is seated onthe valve seat 61, to thereby close the fluid passage 52 (the simulatorpassage 6) in the simulator body 40.

Consequently, the supply of brake fluid to the stroke simulator 5 isstopped; and as a result, the master cylinder apparatus 1 operates as amanual brake, and supplies a desired amount of brake fluid to each ofthe wheel cylinders.

In this state, the fluid pressure in the first fluid pressure chamber 13acts on the rear side of the valve body 62 of the poppet valve 54.Therefore, the poppet valve 54 reliably closes the simulator passage 6,with the aid of the elastic member 64 connected to the lower endthereof. When the BBW system is normally operated, the valve body 62 ofthe poppet valve 54, which is in a standby condition, is suspended at aposition separate from the valve seat 61. Therefore, if the valve body62 is placed in a standby condition for a prolonged period of time, theelastic member 64 will not be subject to deformation or damage, and thesimulator passage 6 can consequently be reliably closed in the event offailure of the system.

When the brake pedal 3 is released, since the spring force of the secondreturn spring 21 is larger than that of the first return spring 20, thesecondary piston 12 is first retracted, thus lowering a fluid pressurein the second fluid pressure chamber 15. Consequently, the brake fluidreturns from the wheel cylinders to the second fluid pressure chamber15, while the brake fluid is supplied from the reservoir 29 through thecup seal 37 to the second fluid pressure chamber 15. The secondarypiston 12 finally abuts against the stopper pin 25. Thus, a retractedposition of the secondary piston 12 is limited, and the secondary piston12 is stopped at its initial position. In this instance, the secondfluid pressure chamber 15 and the reservoir 29 are communicated witheach other through the supply port 33 formed in the cup-like portion 12a of the secondary piston 12, to thereby control the brake fluid in thesecond fluid pressure chamber 15. On the other hand, under the springforce of the first return spring 20, the primary piston 11 returns toits initial position later than the secondary piston 12. Consequently,the first fluid pressure chamber 13 and the reservoir 29 arecommunicated with each other through the supply port 32 formed in thecup-like portion 11 a of the primary piston 11, to thereby control thebrake fluid in the first fluid pressure chamber 13. As described above,the cup seal 35 on a side of the primary piston 11 is configured toprevent the backside flow (FIG. 5). Therefore, no brake fluid issupplied from the reservoir 29 to the first fluid pressure chamber 13during a return stroke of the primary piston 11. Thereafter, when thebrake pedal 3 is operated, the primary piston 11 and the secondarypiston 12 advance again. At this time, an invalid stroke is notgenerated, since the primary piston 11 and the secondary piston 12 areaccurately returned to their initial positions by means of the pistonguide 23 and the stopper pin 25. Therefore, stable braking can beconducted, even in the event of failure of the BBW system.

In this embodiment, the opening/closing means 7 is provided in thesimulator passage 6 directly extended from the cylinder body 10 of themaster cylinder 4 to the stroke simulator 5. Therefore, there is no needto provide an extra valve element inside the secondary piston 12, thusachieving a reduction in size of the secondary piston 12. In thisembodiment, the opening/closing means 7 comprises the poppet valve 54.Therefore, it is unnecessary to use a seal member, with concomitant riskof damage, and the opening/closing means 7 can be stably operated over aprolonged period of time. This markedly improves reliability of theapparatus in the event of failure of the BBW system. Further, the poppetvalve 54 is opened and closed by the rocking lever 55 which movestogether with the secondary piston 12. Therefore, no special drive meansis necessary for operating the poppet valve 54, which simplifies astructure of the opening/closing means 7. Further, in this embodiment,the stroke sensor 8 has a mechanism such that a linear motion of theprimary piston 11 is converted to a rotational motion through engagementbetween the sensor pin 75 and the sensor arm 73. Therefore, the strokesensor 8 is made simple in structure and is reduced in size, whichresults in an overall reduction in size of the entire apparatus.

A structure of the opening/closing means 7 is not particularly limited.Instead of the poppet valve 54 in the above-mentioned embodiment, a tiltvalve, a slide valve or a spool valve may be used.

In the above-mentioned embodiment, use is made of a master cylinder inwhich the second return spring 21 for the secondary piston 12 has alarger spring force than the first return spring 20 for the primarypiston 11. However, this does not limit the present invention. Themaster cylinder may be such that the first return spring 20 for theprimary piston 11 has a larger spring force than the second returnspring 21 for the secondary piston 12. In this case, since the retractedposition of the secondary piston 12 is limited by the stopper pin 25, itis possible to avoid a phenomenon such that when the brake is releasedthe secondary piston 12 travels an excessive distance, i.e., overshoot.

Although only some exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teaching andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

The entire disclosure of Japanese Patent Application No. 2003-341338filed on Sep. 30, 2003 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A master cylinder apparatus comprising: a tandem-type master cylinderadapted to be connected to wheel cylinders through fail-safe valves; anda stroke simulator for ensuring a desired stroke of a brake pedal uponreceiving a brake fluid introduced from a primary-side fluid pressurechamber formed in the master cylinder, wherein: the tandem-type mastercylinder comprises: a cylinder body; a primary piston and a secondarypiston arranged in an axial direction of the cylinder body and beingcapable of sliding movement in the cylinder body; the primary-side fluidpressure chamber, which is defined between the primary piston and thesecondary piston; a secondary-side fluid pressure chamber definedbetween the secondary piston and the cylinder body; a firstcircumferential groove and a second circumferential groove formed in aninner surface of a bore of the cylinder body, the first circumferentialgroove and the second circumferential groove being spaced apart fromeach other in the axial direction of the cylinder body; a first cup sealprovided in the first circumferential groove and adapted to seal anouter circumferential surface of the primary piston; a second cup sealprovided in the second circumferential groove and adapted to seal anouter circumferential surface of the secondary piston; a first returnspring provided in the primary-side fluid pressure chamber so as to biasthe primary piston in a direction for retraction; a second return springprovided in the secondary-side fluid pressure chamber so as to bias thesecondary piston in the direction for retraction; and a limiting meansprovided in the cylinder body so as to limit a retracted position of thesecondary piston.
 2. A master cylinder apparatus according to claim 1,wherein the second return spring for biasing the secondary piston has alarger spring force than the first return spring for biasing the primarypiston.
 3. A master cylinder apparatus according to claim 1, wherein thelimiting means comprises a stopper pin which extends across the bore ofthe cylinder body, the stopper pin being provided in an oblong holeextending through the secondary piston.
 4. A master cylinder apparatusaccording to claim 2, wherein the limiting means comprises a stopper pinwhich extends across the bore of the cylinder body, the stopper pinbeing provided in an oblong hole extending through the secondary piston.